Access VectorDrawableAtlas only on RenderThread

VectorDrawableAtlas dtor can run only on RenderThread, because
it releases GPU resources. It is usually CacheManager, which
destroys the atlas on RenderThread in response to trimMemory.
This CL fixes a rare situation, when a Java finalizer thread
deletes the atlas.

Bug: 137722646
Test: Manual testing on a device
Change-Id: I9d900809c23efaead9aaf16ee3306bc8c7c6b379